On Wed, Apr 07, 2021 at 11:28:48PM -0400, Eric Farman wrote:
On 4/7/21 9:07 AM, Pavel Hrdina wrote:
On Fri, Mar 26, 2021 at 05:25:03PM +0100, Eric Farman wrote:
The introduction of nested cgroups used a little macro virCgroupGetNested() to retrieve the nested cgroup pointer, if one exists. But this macro isn't used when removing cgroups, resulting in some messages:
Mar 25 20:55:17 fedora33 libvirtd[955]: unable to open '/sys/fs/cgroup/machine.slice/machine-qemu\x2d1\x2dguest.scope/': No such file or directory Mar 25 20:55:17 fedora33 libvirtd[955]: Failed to remove cgroup for guest
That directory exists while the guest is running, as it was created by systemd/machined, so the code probably meant to open the libvirt/ subdirectory from that point.
Similarly, there happen to be BPF-related file descriptors that don't get cleaned up in this process too, because they are anchored off the nested cgroup location:
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l 35 [test@fedora33 ~]# virsh create guest.xml Domain 'guest' created from guest.xml
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l 42 [test@fedora33 ~]# virsh shutdown guest Domain 'guest' is being shutdown
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l 37 [test@fedora33 ~]# virsh create guest.xml Domain 'guest' created from guest.xml
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l 44 [test@fedora33 ~]# virsh shutdown guest Domain 'guest' is being shutdown
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l 39
Let's fix this by using the same macro when removing cgroups, so that it picks up the right structure and can remove the associated resources properly.
Fixes: 184245f53b94 ("vircgroup: introduce nested cgroup to properly work with systemd") Signed-off-by: Eric Farman <farman@linux.ibm.com> --- src/util/vircgroup.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
I don't thing this patch is correct. With systemd we would get the same error without the nested cgroup as well. It's because we terminate the qemu process which makes systemd remove the VM root cgroup as well.
I don't experience any problems reverting the blamed patch. The qemu cgroup code doesn't make any distinction about systemd or not; it just calls the virCgroupRemove() to clean up the resources that were set up here during init:
qemuInitCgroup() virCgroupNewMachine() virCgroupNewMachineSystemd() virCgroupNewNested()
The group pointer that's stashed in qemu's struct is that of the machine-qemu...scope group, rather than the nested group, but nothing in the cleanup path touches group->nested. My initial patch is certainly flawed (as you explain below), so maybe something like this is better?
@@ -2615,6 +2615,9 @@ virCgroupRemove(virCgroupPtr group) { size_t i;
+ if (group->nested) + virCgroupRemove(group->nested); + for (i = 0; i < VIR_CGROUP_BACKEND_TYPE_LAST; i++) { if (group->backends[i]) { int rc = group->backends[i]->remove(group);
Not great, since it cleans up the nested group but then still attempts to clean up the machine-qemu...scope group that was setup by systemd. This group wasn't setup by virCgroupV2MakeGroup(), so calling virCgroupV2Remove() seems wrong too. Not sure how to address that.
I'm not sure how this will help. As I've already pointed out calling virCgroupRemove(group) results in calling one or both functions: virCgroupV1Remove() virCgroupV2Remove() Where both of these functions will call virCgroupRemoveRecursively() which will recursively remove all subdirectories including the nested one. So the extra if (group->nested) virCgroupRemove(group->nested); will not help with anything. Looking at the code (I did not test it) it looks like the error message is produced by following this path: qemuProcessStop() qemuRemoveCgroup() virCgroupTerminateMachine() this will make systemd to remove the cgroup virCgroupRemove() virCgroupV2Remove() virCgroupV2DevicesRemoveProg() virCgroupV2DevicesDetectProg() open() Unfortunately we cannot simply ignore ENOENT in the virCgroupV2DevicesDetectProg() because it is used in different places where such error should be reported. What we can do is to introduce new parameter `bool quiet` into virCgroupV2DevicesDetectProg() that would silence the ENOENT error if set to true and we could use it from virCgroupV2DevicesRemoveProg() or something similar. Pavel
This happens only on cgroup controllers managed by systemd. For example if you switch to cgroups v1 where each controller is in separate directory not all controllers supported by libvirt are also supported by systemd. In this case libvirt creates all the cgroups by itself and is responsible to cleanup as well. With this patch we would not remove the VM root cgroups in these controllers. This would affect following controllers:
cpuset freezer net_cls net_prio perf_event
You can verify what happens with cgroups v1 by adding systemd.unified_cgroup_hierarchy=0 to your kernel cmdline.
Neat, thanks for that tip.
Thanks, Eric
Pavel